1. Field of the Invention
The present invention relates to a color image scanning apparatus equipped with a color image sensor including a plurality of line sensors arranged in a sub-scanning direction.
2. Description of the Related Art
In recent years, the commercialization of equipment dealing with a color image has been rapidly advanced with the enhancement of performance of computers and the increment of capacity of memories thereof. In particular, color image scanners for inputting a color static image into a computer have been actively commercialized.
In such color image scanners, image sensors of an on-chip filter system in which color separation filters are provided on a sensor chip have been extensively used because of their properties such as high speed and simplified construction.
Generally, in document reading devices, the enlargement/reduction of images to be read along a main scanning direction is achieved by electrical processing, and the enlargement/reduction of images to be read along a sub-scanning direction is achieved by varying the traveling speed of the image sensor in accordance with the magnification ratio with fixing a drive timing of the image sensors.
However, for example, a line sequential color image sensor, in which image sensors are arranged in three rows in the sub-scanning direction and a color separation filter is provided in each row is used, has problems. That is, in such a line sequential color image sensor, the difference from the time when the color data is previously read to the time when the color data is subsequently read varies depending upon the magnification ratio, so that a color shift cannot be prevented unless the time difference is an integral multiple of the main scanning time.
In order to overcome the above-mentioned problems, Japanese Laid-Open Patent Publication No. 1-109966 has proposed a technique for driving line sensors arranged in parallel at the identical timing and obtaining a signal read from apparently the same position by using interpolating means for performing interpolation of image signals from a plurality of lines in accordance with the magnification ratio and the distance between the respective line sensors.
However, even in the above-mentioned construction, there still remain problems to be solved. That is, the color shift caused by the variation of the sub-scanning traveling speed in accordance with the designated magnification ratio can be reduced by interpolation; however, the color shift cannot be completely eliminated since pixels placed at the identical position in the main scanning direction of each row do not read the identical position of the original.
In addition, it is required to perform interpolation during a period of the output corresponding to one pixel, and the construction of the image sensor becomes more difficult with the increment of speed. Furthermore, since the moving distance of line sensors varies depending upon the magnification ratio of the original, coefficients used for linear operation are different, thereby causing the demand for a high-speed multiplier.